Chapter 11 Intensive Care Unit

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CHAPTER 11 • Intensive Care Unit Imaging

Thoracic Ultrasound Until quite recently, the apparent artifacts on 2D images caused by sonic interactions of tissue and air were disregarded, despite their high diagnos- tic value. Assuming the use of appropriate probe, depth, and ultrasonic frequency range, normal lung, pneumothorax, and edematous, consolidated, and liquid-filled spaces return characteristic sonograms that vary during tidal breathing (Fig. 11-21). Sliding of the visceral pleura at its interface with the fixed parietal counterpart as well as repetitive curvilin- ear lines that fan out parallel to the pleural surface (A lines) is characteristic of normal lung. A limited number of hyperechoic, self-erasing ines orthogo- nal to the pleural plane (B lines or lung rockets) are produced by interstitial lung edema. When B lines are profuse, evenly distributed, and bilateral, alveolar edema likely is present. The homogeneous, sharply demarcated, homogeneous and distinctly opaque sonographic shadow separating the parietal from the visceral pleura indicates an uncomplicated pleural effusion. A sinusoidal profile is usually gen- erated during M-mode US (Fig. 11-22). Measuring a pleural liquid depth of greater than 1.5 cm indicates that needle thoracentesis can be safely performed at that site. US is essential to safely perform needle insertion when the patient is receiving mechanical ventilation or when the effusion is loculated. A pneumothorax is characterized by an echo- free separation between the lung and chest wall. Confirmation is made when a motionless probe detects phasic (tidal) transitions during tidal breath- ing between echo free and lung signatures. This “lung

data relevant to the diagnostic evaluations of shock and its resuscitation. Clearly, the possibilities of obstructive shock (pericardial tamponade, mas- sive PE, and pneumothorax) and certain forms of cardiogenic shock can be interrogated by mea- sures already described. Cardiac filling status is effectively assessed by determining inferior vena cava dimensions (ideally >2 cm), respiratory varia- tions in its diameter, and tendency to collapse with probe compression. Moreover, in decisions made to administer or withhold additional fluids in shock states, the wisdom of doing so may be brought into question by the appearance and subsequent multi- plication of B lines that indicate the development of interstitial and alveolar edema. These may appear before vital signs change or clinical signs develop. A number of protocols have been proposed to guide fluid administration in a variety of shock states by practitioner-driven ACUS (e.g., FALLS). Although all are rational and appear in practice to offer ben- efit, evidence-based validation for any has been limited. Ultrasound may be of considerable value dur- ing cardiopulmonary resuscitation (CPR) as it can be applied during noncompressive periods (pulse checks) to quickly identify potentially reversible causes or perpetuators of cardiac arrest such as extreme hypovolemia, acute cor pulmonale, pericar- dial tamponade, and tension pneumothorax. Failure of the heart to contract despite electrical activity and lengthy period of CPR support portends a dis- mal outcome and strongly supports a decision to abandon the attempt to restore effective spontane- ous circulation.

FIGURE 11-21. Left: A lines ( red arrows ) are the diminishing ultrasonic reverberations of the pleural stripe ( white arrow ) through normal lung tissue. Right: B lines are linear streaks ( yellow arrow ) emanating from the pleural boundary. These indicate increased tissue water, such as caused by edema.